This invention claims the priority based on Japanese Patent Application No. 237745/1995, filed Aug. 22, 1995.
Optical axis-alignment means that each optical axis of optical pars, e.g. a light emitting device, a photodetector, an optical waveguide and so on, is aligned with each optical axis of a lens and an optical fiber on a direct line. First of all, the necessity of optical axis-alignment will be explained.
Semiconductor lasers, photodetectors or optical waveguides used for optical communication are capable of fulfilling their functions only by coupling them with an optical fiber with high efficiency. If the coupling efficiency between an optical part and an optical fiber is low, many disadvantages occur. Low coupling efficiency causes, for example, an extremely high loss of optical energy, a short transmission distance and a low signal/noise ratio (S/N ratio). To surmount these impediments, conventionally each optical part is aligned to an optical fiber at a place which maximizes the coupling efficiency.
Further, when the properties of optical devices are examined, the coupling efficiency between each optical device and an optical fiber and the light wavelength which the device emits at the most suitable position are inspected. High-accurate optical axis-alignment is is indispensable in any case.
Conventional optical axis-alignment techniques, and their disadvantages will be explained by citing a semiconductor laser as an example.
A typical structure of a semiconductor laser will be explained according to FIG. 1. A semiconductor laser chip (1) and a monitoring photodiode (2) are put in positions on a straight line in a package of the semiconductor laser. When the light having a 1.3 .mu.m wavelength is, for example, used for optical communication, the laser chip (1) having a light emitting layer made of InGaAsP is used. The semiconductor laser chip (1) is, for example, fixed to a side surface of a protrusion (23) of a metallic header (3) made of Fe (iron), Fe--Ni (iron-nickel) alloy and so forth, wherein a submount (7) made of AlN is sandwiched between the laser chip (1) and the protrusion (23).
Further, the monitoring photodiode (2) is equipped behind the laser chip (1) in order to keep output of the laser constant. The photodiode chip (2) is fixed to the slanting bottom surface of the header (3) via the ceramic sub-mount (8) made of alumina (Al.sub.2 O.sub.3). Each of the laser chip (1) and the photodiode (2) is connected to some one of a plurality of lead pins (4) respectively with gold wires. The photodiode (2) and the laser chip (1) are able to be electrically connected to outer circuits through the lead pins (4). A cylindrical cap (5) equipped with a spherical lens (6) is fixed on an upper surface of the header (3) with laser welding. The semiconductor laser (1), the photodiode (2) and the head parts of the lead pins (4) are sealed in vacuum by the cap (5). The interior of the cap (5) is filled with an inert gas as to increase the reliability of the devices. The spherical lens (6) installed in the cap (5) plays the role of enhancing the coupling efficiency. In this case, the cap (5) and the spherical lens are unified in one body, but there is another example that a header is sealed in vacuum by a cap equipped with a window of a flat glass board, and a lens is positioned the outside of the window without unifying the cap and the lens. The laser diode (1) emits light by supplying an electric current to the laser chip through the lead pins (4). The light output of the laser diode (1) is monitored by the monitoring photodiode (2). The output of the photodiode (2) is in proportion to the light output. The driving current of the laser diode is controlled as retaining the output of the photodiode at a constant value. An optical fiber (9) is placed at the outside of the semiconductor laser body, and an end of the optical fiber (9) is opposite to the spherical lens (6) with a little space. Each axis of the semiconductor laser chip (1), the photodiode (2) and the spherical lens (6) has already been aligned at the time of producing the device. Here, the optical device group including the semiconductor laser chip (1), the photodiode (2) and the spherical lens (6) is a first optical device, and the optical fiber (9) is a second optical device. The optical axis-alignment proposed by the present invention is identical with aligning the axis of the first device with the axis of the second device (optical fiber). Since the first device can focus light by the spherical lens, the intensity of the light entering the optical fiber (5) is maximized by disposing the end surface of the optical fiber right at the focal point. The alignment is carried out by moving the optical fiber relatively not only in the directions orthogonal to the optical axis (X-Y plane) but also in the directions parallel to the optical axis (Z-axis direction).
An ordinary method of optical axis-alignment will be summarized by referring to FIG. 2. An optical fiber (9) is opposite to a light emitting device (semiconductor laser) (16) with a built-in laser diode (1). The laser emits light by supplying an electric current from a driving electric power source (10). The laser chip (1) emits light at a constant optical power by controlling the driving current by a monitoring photodiode (2). An end of an optical fiber (9) is opposite to the light emitting device (16). The end part of the optical fiber (9) is supported by an XYZ driving system (14). The other end of the optical fiber (9) is connected to a light power meter (11) that is connected to a controller (12). The controller (12) gives a controlling signal (13) to the XYZ driving system (14). The semiconductor laser (16) emits light at a constant output, and the optical power passing through the optical fiber (9) is monitored by the light power meter (11). The XYZ driving system (14) makes the optical fiber (14) move in X and Y directions vertical to the optical axis (XY plane) and in Z direction parallel to the optical axis, and seeks the position where the coupling light intensity Pf is the largest. It is feasible to obtain the coupling light intensity Pf at an arbitrary position of the optical fiber (14) due to the combination of the light power meter (11), the controller (12) and the XYZ driving system (14). The coupling light intensity Pf is maximized at the point where the laser light is focused by the lens. When the position where Pf is the largest is found out, the end part of the optical fiber is maintained at the position. Inspection data are obtained by measuring the output power and the wavelength from the fiber. These procedures mean optical axis-alignment for inspecting a semiconductor laser and so forth.
Other purpose of an axis-alignments is a step of production. There is a pigtail type laser module, which is produced by fixing a ferrule of a fiber to a housing of a semiconductor laser at the position of maximizing Pf (the most suitable coupling position). This is an axis-alignment for accomplishing the most effective coupling between a light emitting device and an optical fiber. Otherwise, when a high output is not necessary or is troublesome, an end part of the optical fiber is intentionally shifted from the position where Pf is maximized, whereby a predetermined intensity of light is possible to enter the optical fiber. The properties of the laser module can be inspected, or the laser module is fixed at the shifted position.
As explained above, when optical devices are examined or optical devices are coupled with an optical fiber, the most suitable coupling position (the position of maximizing Pf) is firstly sought by moving the optical fiber in X and Y directions orthogonal to the optical axis and in Z direction parallel to the optical axis. Hence, finding out the most suitable coupling position is a primarily necessary operation. There is, however, a difficulty of wasting so much time for seeking the most suitable coupling position. The reasons why it takes much time to find out the most suitable coupling position will be explained.
One reason is that both a spot size Q of laser light focused by the lens and a core size W of an optical fiber receiving the light are small. The largest power of incident light accrues from the coincidence between the focus point Q and the optical fiber core W. Since such the small focus point Q is sought by moving the small core W, the distance to the most suitable coupling position would be extremely long from the starting point of search. For example, when the semiconductor laser of a 1.3 .mu.m wavelength is focused, its spot size becomes several .mu.m. In the case of a quartz-type single mode fiber, its core diameter is about 10 .mu.m, which is absolutely small. Consequently, the optical fiber must be aligned with a high-accuracy in .mu.m order, because the optical fiber must be put in the position where the coupling efficiency of the light focused by the lens is maximized. This procedure is extremely difficult.
FIG. 3 and FIG. 4 are graphs for explaining an example of the alignment tolerance between the semiconductor laser and the optical fiber. FIG. 3 is a graph showing the change of light power received by the optical fiber in the case of moving the optical fiber to the axial direction (Z-direction) from the most suitable coupling position. The abscissa indicates the distance in Z-direction from the most suitable coupling position. Z=0 means the most suitable position. As the position of the optical fiber separates from the most suitable coupling position, Z showing the distance moves to the right direction on the abscissa. As shown in FIG. 3, the range of Pf from 0 dB to -0.5 dB extends to about 160 .mu.m in length. It is proved that the tolerance in Z-direction is large, so that the axis-alignment in Z-direction is easy to passable.
FIG. 4 is a graph showing the change of the light power received by the optical fiber while the optical fiber moves in X-direction or in Y-direction from the most suitable coupling position. The abscissa indicates the distance in X-direction or in Y-direction from the most suitable coupling position. The most suitable coupling position is represented by X=0 and Y=0. The range of Pf from 0 dB to -0.5 dB has a short length of .+-.2.5 .mu.m. Hence, the axis-alignment in X and Y directions needs a high accuracy of a tolerance less than 5 .mu.m. It is known that the axis-alignment in X and Y directions is extremely difficult.
Second reason is that the positional accuracy in fabricating devices, e.g. semiconductor laser and so on, is low. Even if the semiconductor laser is set in a measuring device, and the optical fiber is arranged at the position that seems to be nearly just above the lens; the deviation from the most suitable coupling position of the optical fiber is about plus or minus several hundreds of .mu.m square, that indicates an initial gap (deviation) between the set-position of the semiconductor laser and the initial position of the optical fiber. In addition to this gap, there exists another deviation of the chip in the package of the semiconductor laser. The gap, that is generated between the initial position of the optical fiber and the most suitable coupling position, is so large that the axis-alignment requires high accuracy. Actual axis-alignment procedure becomes a hard operation, because the most suitable coupling position of less than a 1 .mu.m round must be searched in a square of several hundreds of .mu.m. Here, a "search region" is defined to be the region of moving the optical fiber for seeking the most suitable coupling position. The search region expands in a several hundreds of .mu.m square. It takes an enormous time to examine the whole search region from corner to corner. Therefore, some improvements were proposed in order to shorten the searching time.
FIG. 5 shows a conventional method of searching the most suitable coupling position. An initial position (A.sub.0) of the optical fiber is set up as the focus point (A.sub.2 point) of the semiconductor laser certainly exists anywhere in the whole search region (30) of, for example, a 500 .mu.m square whose center is the initial position (A.sub.0). The region capable of sensing a non-zero amount of laser light is called "scanning region (31)". The end part of the optical fiber is held to move freely. The optical fiber is positioned at A.sub.0 point at the beginning. The sensing end of the optical fiber is moved by the XYZ driving system (14), keeping watch on the incoming light from the optical fiber by the light power meter (11) installed in another end of the optical fiber. The movement of the optical fiber starts from the center point A.sub.0 of the search region (30), and proceeds along the routes (33), (34), (35), and (36) of a square type helical line. As long as the incident light to the optical fiber is almost zero, the optical fiber continues to move along the helical line, increasing the distance between the point A.sub.0 and the route little by little. It is assumed that the non-zero light power enters the optical fiber at the moment of passing through a certain point A.sub.1 of the scanning region (31). The helical movement of the optical fiber is finished, and the optical fiber runs in the direction of increasing light power, taking a zigzag course (37) and changing a direction of the movement slightly. Since the movement in the scanning region (31) is guided by the controller, observing the incident light power Pf, it is feasible to reach the most suitable coupling point A.sub.2 in a short time. The maximum incident light power Pf.sub.max is detected at the point A.sub.2. The helical movement in the period of detecting no light is called a "rough axis-alignment", and the zigzag movement, that is carried out after non-zero light power is detected at point A.sub.1, is called a fine axis-alignment.
FIG. 6 is a graph showing the relation between the incident light power and the seeking time. There exists a period where the incident light power is zero at the beginning. This period is correspondent to the term of the helical movement of the optical fiber. At T.sub.1 time from the beginning of the axis-alignment, the limited light power Pf.sub.0 is firstly detected, which corresponds to point A.sub.1. T.sub.1 indicates the time of rough axis-alignment. After that, the optical fiber rises the light power gradient, so that the light power is rapidly increased. At the time of (T.sub.1 +T.sub.2), the end of the optical fiber arrived at the most suitable coupling position point A.sub.2 where Pf equals to Pf.sub.max, i.e. Pf.=Pf.sub.max. The axis-alignment explained by referring to FIG. 5 is limited in XY plane. Actually, it is necessary to practice the axis-alignment in Z direction. There exists an axis-alignment of method of aligning in three directions X, Y and Z at the same time. Further, another axis-alignment method is proposed in which the alignment in Z direction is done after the search of the most suitable coupling position in XY plane has finished. In these systems, it takes one or two minutes for the time T.sub.1 of rough axis-alignment. The time T.sub.2 for the fine axis-alignment is 30 seconds to 60 seconds. For increasing the throughput of producing semiconductor lasers, the time (T.sub.1 +T.sub.2) for the axis-alignment is required to be more shorter. It is necessary to shorten the time of axis-alignment and to reduce expenditures.
There is another serious obstacle. A few semiconductor lasers incapable of emitting light are sometimes mixed in the products. These defects must be excluded. The judgement whether a semiconductor laser is defective or not cannot be done in the period T.sub.1 of rough axis-alignment, because no light enters. As mentioned before, the optical fiber travels along the helical routes, however, even if the helical route from A.sub.0 becomes much longer, no light can be detected by the defective laser. Hence, the rough axis-alignment time T.sub.1 becomes excessively long for the defective ones. When the light from the semiconductor laser is not detected even if it takes a certain time T.sub.1 for the rough axis-alignment, the semiconductor laser is judged to be rejected. Therefore, the axis-alignment comes to an end. In this method, the scanning begins from the no light condition, so that the detection of defective products requires an excessive long time. A new method, in which defective products are immediately detected and the axis-alignment procedures are eliminated for the useless products, is strongly desired. Visible light lasers suffer from the same problems as mentioned above. When a visible light semiconductor laser is used, an initial position can be set by an operator watching the light of the visible light semiconductor laser. Further, it is feasible to detect non-light devices. When an infrared light laser (emits the light with a wavelength of more than 0.9 .mu.m) is, however, used, it is not easy to set the laser at an initial position. When semiconductor lasers emitting light with a wavelength of 1.3 .mu.m and a wavelength of 1.5 .mu.m are used, an initial position can not be established by observing with the eye.